In conventional electric power steering systems, operation of an electric motor is controlled in accordance with torque applied to a steering wheel of a vehicle by a driver. Specifically, the torque is detected by a torque sensor assembled in a steering shaft of the steering wheel, for example, and outputted to an electronic control unit (ECU). The ECU controls the operation of the electric motor in accordance with a detected torque.
The EPS is mounted recently not only in light-weight vehicles but also in normal passenger vehicles. The number of vehicles in which steering operation of a steering wheel is power-assisted by the EPS is remarkably increasing. If the EPS fails to power-assist the steering operation, the load to the driver in steering the steering wheel suddenly changes. To counter this problem, various controls are proposed.
According to EPS of the following patent documents 1 to 7, even when a failure arises in the EPS, the power-assisting operation of the EPS is not stopped entirely but the electric motor is continued to be operated based on a control method, which is predetermined to avoid that the load to the driver in steering the steering wheel changes suddenly.
Since the controls proposed in the patent documents 1 to 7 are primarily for avoiding the sudden change in the steering load by the continuation of the power-assisting operation as an emergency measure. The assisting power in this instance is limited to a rather low assisting-power. Thus, it is necessary to gradually reduce the assisting power until it becomes null. It is required therefore to avoid stopping of the power-assisting operation by maintaining the assisting power, even when the EPS fails.    Patent document 1: JP 9-58505 A    Patent document 2: JP 2002-255054 A    Patent document 3: JP 2005-193834 A    Patent document 4: JP 2004-196128 A    Patent document 5: JP 2005-271860 A    Patent document 6: JP 2005-289190 A    Patent document 7: JP 2007-283891 A